174 EXPERIMENTS ON AMOUNT OF EXHALATION. 



264. Experiments of this last kind may be very easily 

 performed by any one who has command of a pair of scales 

 adapted to weigh small substances; and it is well that the 

 student should avail himself of such opportunities, for learning 

 how to " put Nature to the question" in matters of this eimple 

 character, in order to cultivate habits of accuracy and caution, 

 which are useful in every condition of life. Let him take several 

 leaves of different plants, such, for example, as the Vine, Oak, 

 Elm, Beech, Lime, Apple, Pear, weigh them separately, and 

 estimate as nearly as he can the comparative surface presented by 

 each. He should then place their footstalks in glasses or bottles 

 of equal size, into which lias been poured a certain weight of 

 water, carefully ascertained to be the same in each; and he 

 should place all these in similar circumstances for a certain time ; 

 having also a corresponding glass without a leaf, in order to esti- 

 mate the amount of fluid lost from the surface of the water by 

 evaporation. By ascertaining how much had been absorbed by 

 each leaf, and the weight each had gained, he would thus be 

 easily enabled to calculate the quantity it must have exhaled ; 

 and then, by comparing this with the extent of the surfaces of 

 the different leaves, he would estimate the proportional rapidity 

 of the process in the various species he had chosen, care having 

 been taken to select, in the first instance, trees in equal stages 

 of growth, and leaves of a similar degree of freshness and deve- 

 lopment. 



265. The watery vapour which is constantly, though insen- 

 sibly, given off from the skin of Animals, is liable to accumulate 

 in drops, and to form sensible perspiration, when from any cause 

 it exceeds in quantity that which the air can carry away ; either 

 in consequence of an increased secretion or separation of it from 

 the blood (as when a person exerts himself in warm weather), or 

 from the atmosphere being already so loaded with dampness, 

 that it cannot contain any additional moisture (ANIM. PHYSIOL., 

 . 371, 372 ) In the same manner, some Plants exhale so 

 rapidly at sunrise, when the heat of the air is not sufficient to 

 enable it to carry off the disengaged moisture, that the fluid 

 accumulates in drops at the points of the leaves, and has been 



